This invention relates to a heat-curable photosensitive composition suitable in forming an interlayer insulating film in thin-film magnetic heads.
In the prior art, heat-curable positive photosensitive compositions are known, for example, from JP-A 3-223702 and JP-A 7-140648. These photosensitive compositions, however, are less resistant to chemicals and heat and insufficient in resolution. There is a need for a heat-curable photosensitive composition which is improved in these properties.
An object of the invention is to provide a heat-curable photosensitive composition which when subjected to patterning exposure and heating, features a high resolution and an effective progress of curing reaction and forms a pattern having improved chemical resistance and heat resistance.
The inventor has found that blending a base-soluble resin in the form of a cresol and/or xylenol novolac resin comprising recurring units of the following general structural formula (1) which has been partially 1,2-naphthoquinonediazido-4- or 5-sulfonate esterified and having a weight average molecular weight of 1,000 to 10,000 with an epoxy compound of the following general structural formula (2) gives a heat-curable photosensitive composition which can be effectively micro-patterned through an exposure step and form, subsequent to heating, a film which is so improved in solvent resistance and heat resistance as to be suitable as an insulating film or the like.
Accordingly, the invention provides a heat-curable photosensitive composition comprising 100 parts by weight of a resin comprising recurring units of the following general structural formula (1) and having a weight average molecular weight of 1,000 to 10,000, 5 to 30 parts by weight of an epoxy compound of the following general structural formula (2), and a solvent therefor. 
Herein R is hydrogen or a 1,2-naphthoquinonediazido-4- or 5-sulfonate residue, the proportion of the sulfonate residue being 2.5 to 27 mol % of the R groups, and m is an integer of 0 to 3. 
Herein X is hydrogen or an alkyl group of 1 to 6 carbon atoms, Y is a trivalent chainlike or cyclic aliphatic group of 1 to 6 carbon atoms or trivalent aromatic group of 6 to 9 carbon atoms, the letter a is a positive number of 1 to 20, each of b and c is a positive number of 1 to 3, b+c is equal to 4, and d is a number of 0 to 3.
The heat-curable photosensitive composition of the invention contains a resin comprising recurring units of the following general structural formula (1). 
Herein R is hydrogen or a 1,2-naphthoquinonediazido-4-sulfonate or 1,2-naphthoquinonediazido-5-sulfonate residue. The proportion of the sulfonate residue is 2.5 to 27 mol %, preferably 3.0 to 20 mol % of the R groups. If the percent esterification is less than 2.5 mol %, the effect of inhibiting the dissolution of unexposed areas in a base developer becomes poor, failing to form a positive pattern. If the percent esterification is more than 27 mol %, the dissolution of exposed areas in a base developer becomes poor, resulting in a poor resolution. The letter m is an integer of 0 to 3.
The resin should have a weight average molecular weight (Mw) of 1,000 to 10,000. With a Mw of less than 1,000, the film as post-cured may be less resistant to heat. A resin with a Mw of more than 10,000 may be low in resolution and sensitivity during pattern formation.
Also contained in the photosensitive composition of the invention is an epoxy compound of the following general structural formula (2). 
Herein X is hydrogen or an alkyl group of 1 to 6 carbon atoms such as methyl, ethyl, propyl, isopropyl, n-butyl, tert-butyl, pentyl and hexyl. Y is a trivalent chainlike or cyclic aliphatic group of 1 to 6 carbon atoms or trivalent aromatic group of 6 to 9 carbon atoms, examples of which are given below. 
The letter a is a positive number of 1 to 20, preferably 1 to 10. Each of b and c is a positive number of 1 to 3, and b+c is equal to 4. The letter d is a number of 0 to 3.
The epoxy compound should preferably have a weight average molecular weight (Mw) of 100 to 10,000 and preferably 300 to 5,000.
The amount of the epoxy compound added is 5 to 30 parts by weight per 100 parts by weight of the resin. With less than 5 parts of the epoxy compound, the film as post-cured lacks solvent resistance and heat resistance. A composition with more than 30 parts of the epoxy compound is less effective in forming a pattern upon radiation exposure.
The heat-curable photosensitive composition is obtained by dissolving the resin and the curing agent (epoxy compound) in a suitable solvent, typically to a solid concentration of about 10 to 60% by weight. Examples of the solvent used herein include ethylene glycol monoalkyl ethers and acetates thereof, propylene glycol monoalkyl ethers and acetates thereof, diethylene glycol mono or di-alkyl ethers, alkyl lactates, alkyl alkoxypropionates, ketones such as methyl isobutyl ketone and cyclohexanone, and acetates such as butyl acetate. These solvents may be used alone or in admixture.
If desired, surfactants including nonionic, fluorochemical and silicone surfactants may be added for improving the applicability of the composition.
It is not critical how to use the heat-curable photosensitive composition in forming a resist pattern through radiation exposure. Pattern formation can be conducted by any conventional process. After the formation of a resist pattern, it is heat treated whereby a thermoset pattern is obtained.
More particularly, the composition in solution form is spin coated onto a substrate such as a silicon wafer, and prebaked thereto. The coating is exposed to ultraviolet radiation by an exposure means such as a proximity aligner or stepper, then developed and rinsed, forming the end resist pattern. The developer used herein includes aqueous solutions of inorganic bases such as sodium hydroxide and sodium carbonate and aqueous solutions of organic bases such as triethanolamine, tetramethylammonium hydroxide and choline. The thus formed resist pattern is heat treated on a hot plate or in a dryer at a temperature of about 130 to 200xc2x0 C., preferably about 150 to 200xc2x0 C. for about 1 to 30 minutes. This results in a thermoset pattern having improved chemical resistance and heat resistance.
There has been described a heat-curable photosensitive composition from which a pattern featuring improved solvent resistance and heat resistance can be formed at a high sensitivity and resolution by a simple process. The pattern is suitable as an interlayer insulating film for use in thin-film magnetic heads.